The broad, long-term objective of this proposal is to investigate the local regulation and mechanisms of oxygen (O2) transport through microvascular networks in resting and contracting striated muscle. The specific aims are to investigate, both experimentally and theoretically, the O2 transport characteristics of arterioles, capillaries and venules and their diffusive interactions before, during and after muscle contraction Oxygen transport in arteriolar and capillary networks depends on geometric (vessel diameter, length, proximity to other microvessels and branching pattern), hemodynamic (red blood cell (RBC) velocity, microvessel hematocrit) and oxygenation (diffusion and solubility coefficients, oxygen tension (PO2) and hemoglobin oxygen saturation (SO2)) factors. Current analysis of experimental data has led to the formulation of the following hypotheses that will be tested during the project: (1) the major site of O2 exchange shifts from the arterioles to the capillaries on going from rest to exercise; (2) with increasing severity of exercise, capillary recruitment occurs initially, followed by increases of RBC velocity, RBC lineal density and inlet capillary SO2; (3) heterogeneity of hemodynamic parameters decreases with exercise, whereas heterogeneity of end capillary S02 increases with exercise; (4) during severe contractions 02 transport is diffusion-limited and a major part of the resistance to O2 transport is located inside the capillaries; and (5) capillaries with low RBC velocity and high lineal density deliver more O2 to tissue than capillaries with high RBC velocity and low lineal density. Experimental studies will employ intravital microscopy of the hamster retractor muscle. Microspectrophotometric methods will be used to measure RBC velocity, microvessel hematocrit and SO2 in arterioles. Computer-aided video densitometric methods will be used to perform similar measurements in capillaries. Transmural and tissue P02 gradients will be measured with sharpened O2 microelectrodes. Current mathematical models of O2 transport will be extended to include more accurate descriptions of 02-hemoglobin kinetics and three-dimensional vascular geometry to account for diffusive interactions between capillaries and other microvessels. The results of this project should provide needed information to delineate the local regulatory mechanisms involved in the response of the O2 transport system to the physiological stress of muscle contraction.